Sand cores are shaped solid aggregates of sand which are used in foundries for making metal castings. The sand cores are usually placed in a mold to define the internal recesses of the casting. When molten metal is poured over the sand core, the rapid rise in temperature causes thermal expansion of the sand mass, often resulting in the formation of cracks in the core. These cracks allow molten metal to penetrate the core and form fin-shaped imperfections on the surface of the casting which are known in the art as veins.
Various additives have been proposed for reducing the cracking of sand cores during casting. For example, Industrial Gypsum Co.'s U.S. Pat. No. 5,911,269, the disclosure of which is hereby incorporated by reference for all purposes, describes adding a lithia-containing additive, such as α-spodumene, to sand cores to reduce thermal expansion defects. The lithia-containing material is added to the sand core in an amount sufficient to provide about 0.001% to about 2.0% lithia. The patent speculates that α-spodumene absorbs free silica during casting to form β-spodumene, which is said to have extremely low thermal expansion.
ICG Technologies, Inc. (Milwaukee, Wis.) markets a lithia-containing anti-veining agent under the name VEINSEAL® 14000. This material comprises 60-70% by weight SiO2, 10-20% by weight Fe3O4, 15-25% by weight Al2O3, 10-25% by weight TiO2 and 2-5% by weight Li2O. While effective at reducing veining, this material is expensive, costing about $720 per ton.
U.S. Pat. No. 6,972,302, the disclosure of which is hereby incorporated by reference for all purposes, describes the addition of Fe2O3 to sand cores to reduce the amount of VEINSEAL® 14000 needed to eliminate thermal expansion of sand cores and the formation of vein defects during metal casting. The patent states that the minimum effective concentration of VEINSEAL® 14000, when used alone, is 5% by weight of the sand cores. However, it is said the addition of 1% by weight of Fe2O3 allows the amount VEINSEAL® 1400 to be reduced to about 1% to about 3.5% by weight of the sand cores, resulting in substantial cost savings.
Despite these advances, there is still a need in the art for additives for inclusion in sand cores which reduce or eliminate surface defects (veining) in metal castings. It is an object of the present invention to provide anti-veining additives which substantially reduce or eliminate cracking of sand cores during metal casting and, as a result, prevent or reduce the appearance of veins on the surface of the metal cast. It is another object of the invention to provide effective anti-veining additives that are more cost effective than those currently available.
The foregoing discussion is presented solely to provide a better understanding of nature of the problems confronting the art and should not be construed in any way as an admission as to prior art nor should the citation of any reference herein be construed as an admission that such reference constitutes “prior art” to the instant application.